1. Field of the Invention
The present invention relates in general to pressure sensors, and more particularly, to pressure sensors of a type which can detect the pressure in a combustion chamber of an internal combustion engine or the like.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional pressure sensor of the above-mentioned type will be outlined, which is a combustion pressure sensor disclosed in Japanese Utility Model First Provisional Publication No. 60-535.
The sensor comprises generally a casing, a diaphragm integrally mounted to the casing in a manner to axially move in accordance with an external pressure applied thereto, a transmission rod received in the casing and having one end engaged with the diaphragm, and a piezoelectric transducer having the other end of the transmission rod connected thereto. Thus, the piezoelectric transducer outputs a voltage signal which is varied in accordance with the external pressure applied to the diaphragm.
The piezoelectric transducer used in the above-mentioned pressure sensor is of a so-called "d33" type, which will be described in the following with reference to FIGS. 6 and 7 of the accompanying drawings.
In FIG. 6, denoted by numeral 1 is a transducer proper which is constructed of a piezoelectric material such as lead titanate or the like. The transducer proper 1 has upper and lower surfaces on which respective electrodes 1A and 1B of conductive paste are provided. Although not shown, the transducer proper 1 has a portion to which the transmission rod is connected, and the electrodes 1A and 1B are electrically connected through conductive paths to a control unit. At the time of production of the transducer proper 1, the same is subjected to a high electric field through the electrodes 1A and 1B, so that spontaneous polarization is oriented to cause the polarization axis "P" to extend axially as shown. Thus, when a pressure (or stress) is applied to the transducer proper 1 from the stress axis "F" which is in parallel with the polarization axis "P", a certain distortion appears in the transducer proper 1 and thus an electric charge (viz., voltage signal) having a magnitude corresponding to the distortion is outputted through the electrodes 1A and 1B in the direction of the signal axis "V".
In practical use, the pressure sensor is fixed to a cylinder head of an internal combustion engine having the diaphragm exposed to a combustion chamber of the engine. Thus, the combustion pressure in the combustion chamber moves the diaphragm and thus presses through the transmission rod the transducer proper 1. Thus, the pressure sensor outputs a voltage signal which is varied in accordance with the combustion pressure of the engine.
However, due to its inherent construction, the above-mentioned pressure sensor has the following drawback.
That is, during operation of the engine, the heat in the combustion chamber is transmitted through the casing to the transducer proper 1 to heat the same. However, as is understood from the graph of FIG. 7, the characteristic (viz., output) of the transducer proper 1 is varied in accordance with the temperature, which thus needs a temperature compensation when measuring the combustion pressure with accuracy.
In view of this fact, hitherto, a measure has been employed in which a temperature sensor, such as thermocouple, thermistor or the like, is fixed to the pressure sensor to detect the temperature of the transducer proper 1, and a control unit is so designed as to correct the output (viz., pressure representing signal) from the pressure sensor based on the temperature detected by the temperature sensor. However, even in such measure, usage of two sensors (viz., pressure sensor and temperature sensor) causes not only a complicated construction of the control unit, but also a bulky construction of the pressure sensor.